The present invention relates to a wiring board assembly in which a wiring board is reinforced by a reinforcing member to prevent warpage or bending of the wiring board, and a method of manufacturing the wiring board assembly.
A semiconductor integrated circuit chip (hereinafter just referred to as “IC chip”) has various applications such as a microprocessor for a computer. There is a tendency to provide an increasing number of terminals on the IC chip with a smaller terminal pitch for higher speed and performance of the IC chip. In general, a plurality of terminals are arranged closely in an array on a bottom side of the IC chip and are connected by flip-chip bonding to terminals of a motherboard. The direct mounting of the IC chip on the motherboard is however difficult due to a great difference between the terminal pitch of the IC chip and the terminal pitch of the motherboard. It has thus been common practice to produce a semiconductor package by mounting the IC chip on a chip mounting wiring board, and then, mount the semiconductor package on the motherboard as proposed in Japanese Laid-Open Patent Publication No. 2002-026500.
The IC chip is generally formed of a semiconductor material such as silicon having a thermal expansion coefficient of about 2.0 ppm/° C. to 5.0 ppm/° C. On the other hand, the chip mounting wiring board is often formed of a resin material having a higher thermal expansion coefficient than that of the semiconductor martial of the IC chip. As one type of the resin wiring board, a so-called “build-up wiring board” is already put to practical use. The build-up wiring board includes a core substrate and a plurality of resin insulation layers and conduction layers alternately laminated as build-up layers on top and bottom sides of the core substrate. The core substrate is commonly formed of a resin material such as a resin-impregnated glass fiber material (e.g. a glass/epoxy resin) and made much larger in thickness than the build-up layers, thereby having high rigidity to function as a reinforcement in the wiring board. Further, electrical wiring (e.g. through-hole conductors) is formed through the core substrate for electrical connection between the conduction layers on the top and bottom sides of the core substrate.
In recent years, high-frequency signals have been applied to the IC chip for high-speed operation of the IC chip. In this case, the mounting of the IC chip on the build-up wiring board results in a transmission loss of high-frequency signal or a circuit malfunction and causes interference with the high-speed operation of the IC chip due to high inductance of the electrical wiring in the core substrate. As a solution to such a problem, Japanese Laid-Open Patent Publication No. 2002-026171 proposes a coreless wiring board that has no core substrate of relatively large thickness. In the absence of the core substrate, the total wiring length of the coreless wiring board becomes shortened to reduce the transmission loss of high-frequency signals and to enable the high-speed operation of the IC chip. However, the absence of the core substrate leads to a smaller thickness and lower rigidity of the wiring board. It is thus likely that, when solder bumps for flip-chip bonding get cooled, the coreless wiring board will be warped to its chip mounting side under the influence of a thermal stress due to a difference in thermal expansion coefficient between the chip material and the wiring board material. The warpage or bending of the wiring board becomes a cause of a crack in the joint between the IC chip and the wiring board or an open failure in the IC chip and, by extension, a deterioration in yield rate and reliability of the semiconductor package.
In order to solve the above problem, there has been proposed a semiconductor package 100 that includes a resin wiring board 101, an IC chip 106 mounted on a chip mounting surface 102 of the resin wiring board 101 and a frame-shaped stiffener 105 (as a reinforcing member) fixed to the chip mounting surface 102 of the resin wiring board 101 with the IC chip 106 exposed through an opening of the stiffener 105 as shown in FIG. 39. The stiffener 105 may alternatively be fixed to a bottom surface 103 of the resin wiring board 101. When the stiffener 105 is formed of e.g. a metal material having higher rigidity than that of the resin wiring board 10, however, the resin wiring board 101 may be warped or bent under the influence of a thermal stress due to a difference in thermal expansion coefficient between the resin wiring board 101 and the metal stiffener 105.